1. Field of the Invention
The present invention relates to a motor having a structure in which ball bearings rotatably holding a shaft are fixed to end brackets with adhesive respectively.
2. Description of the Related Art
Conventionally, there are motors having a structure in which a shaft with a rotor member fixed thereon is held by ball bearings. In the conventional motor, a pair of ball bearings are fixed with adhesive to bearing holding portions formed in a pair of end brackets which are fixed to both ends in the axial direction of a stator body having a stator core. In the motor having this kind of structure, an outer ring of one of the bearings is arranged to move in the axial direction of a shaft while the adhesive cures in order to absorb a dimensional error of each member and absorb both an expansion and a contraction of each member during the curing of adhesive or absorb the expansion and contraction of adhesive itself. It has been conventionally considered that there is no problem with this kind of structure after curing of the adhesive.
It has been difficult, however, to minimize the variance of the application quantity of the adhesive, and sometimes peeling off of the applied adhesive layer takes place. As mentioned above, when a structure in which one of the bearing is fixed while the outer ring of the other bearing is allowed to move in the axial direction, the bearing may thrust as a result of the peeling off of the adhesive layer, resulting in the thrust movement of the shaft. Especially when a motor is used under high speed and high response conditions, and the vibration added to the motor increases, the vibration tend to cause the adhesive layer to peel off.
If the motor is provided with an optical encoder which has a rotating plate connected to a shaft of the motor, the thrust movement of the shaft causes the rotating plate of the optical encoder to move in the axial direction of the shaft. This causes miscounting of the encoder or causes the rotating plate of the encoder to touch other members in circumference. As a result, the encoder may break in the worst case. Especially the miscounting problem is a significant problem as the precision of the encoder is advanced.
It is an object of the present invention to provide a motor having a structure to prevent the peeling off of the adhesive layer for fixing the bearings.
It is another object of the present invention to provide a motor in which a bearing is allowed to move in the axial direction during the curing of adhesive while a certain pressure is given to the bearing, and yet the bearing is mechanically stopped from moving along the axial direction after the adhesive has cured.
It is still another object of the present invention to provide a small-sized motor with high capability in anti-vibration.
It is yet another object of the present invention to provide a method for manufacturing a motor having a structure to prevent the peeling off of the adhesive layer for fixing the bearings.
The motor which the present invention intends to improve comprises a stator body and a rotor. The stator body has a stator core. The rotor has a shaft and a rotor member which is fixed on the shaft and is disposed inside the stator core and yet opposite to the stator core. The motor has a first and a second end brackets which are fixed at the end portions in the axial direction of the stator body and have a bearing holder respectively. The motor has a first and a second ball bearings including an inner ring and an outer ring respectively. The inner rings are forcedly fitted onto the shaft and the outer rings are joined to the respective bearing holder portion of the end bracket with adhesive. Generally, the stator core is provided with stator windings, and permanent magnets are used as the rotor member. The ball bearing comprises, in general, an inner ring, an outer ring, and a plurality of rolling members disposed between the inner ring and the outer ring. In a typical structure, the shaft is provided with a first and a second abutting portions (or step portions) which abut the inner rings of the first and the second ball bearing to prevent the inner rings of the first and the second ball bearings from moving toward each other.
The bearing holder portion of the first end bracket is so constructed that the first ball bearing may be prevented from moving mechanically in the axial direction with respect to the first end bracket. More particularly, the bearing holder portion for the first ball bearing comprises a first ball bearing fitting space into which the first ball bearing is fitted and a bearing coming-off preventing structure which prevents the first ball bearing fitted into the first ball bearing fitting space from coming off.
In the present invention, the bearing holder portion of the second end bracket is formed with a second ball bearing fitting space into which the outer ring of the second ball bearing is fitted and a pressure-giving structure receiving space formed continuously to the second ball bearing fitting space and receiving a pressure-giving structure which gives a pressure to the outer ring of the second ball bearing toward the first end bracket. And a spring member is used as a pressure-giving structure. For example, a leaf spring may be used as the spring member. The leaf spring may have an annular form in which crests (or tops) and troughs (or bottoms) alternately appear along the outer ring of the second ball bearing. When such an annular leaf spring is used, the spring is disposed in a compressed state between the outer ring of the second ball bearing and the inner wall surface of the second end bracket. With the annular leaf spring, it is easy to set a spring member in a predetermined position and furthermore it is possible to give an almost balanced pressure to the second ball bearing, thus the assembly precision of the motor can be increased.
Moreover, in the present invention, the second end bracket is provided with a stopper which allows the outer ring of the second ball bearing to move in such a direction as to compress the spring member during the curing of adhesive, but prevents the outer ring of the second ball bearing from giving pressure to the spring member after the adhesive has cured. Adhesive may be any type as far as a given joint strength is acquired. From the manufacturing point of view, however, heat curing adhesive is generally used. During the manufacturing of a motor, the spring member absorbs variance in working precision of the shaft, the first and the second ball bearings as well as the first and the second bearing holder portions by changing the shape of the spring member. Also the spring member absorbs an expansion and a contraction of the adhesive and parts during the curing of the adhesive. After the adhesive has cured, the spring member does not have any specific function. If peeling off of the adhesive layer takes place due to vibrations and the like after the adhesive layer has been formed as a result of the curing of the adhesive, the moving of the second ball bearing may be curbed a little because of the presence of this spring member. However, the spring member only cannot prevent the second ball bearing from moving in the axial direction after the peeling off of the adhesive layer. After the curing of the adhesive, the stopper having the function as mentioned above, will prevent the outer ring of the second ball bearing from moving in such a direction as to compress the spring member. Thus the second ball bearing can be prevented from moving much in the axial direction due to the presence of the stopper, even if the adhesive layer has peeled off. When an encoder or the like is mounted on the shaft of the motor, the above-mentioned structure will prevent encoder""s poor performance in measuring precision or breakage of the encoder due to the moving of the shaft in the axial direction.
For the stopper having the function mentioned above, many kinds of things may be used. For example, when an annular leaf spring is used as the spring member, the stopper may consist of more than one screw holes which extend through the second end bracket in the axial direction toward the second ball bearing and more than one screw members which are inserted into the screw holes and whose tip end portions abut the trough (or bottom) portions of the leaf spring or the outer ring of the second ball bearing. When this kind of configuration is adopted, the screw members are loosened (or removed) until the adhesive cures. After the adhesive has cured, by tightening the screw members to let the tip end portions of the screw members abut the trough (or bottom) portions of the leaf spring or the outer ring of the second ball bearing, the moving of the second ball bearing in the axial direction can be prevented. When screw members are used as the stopper, it is possible to adjust the pressure given by the screw members to the second ball bearing in order to prevent too much pressure from being given to the second ball bearing with the screw members. Especially when the tip end portions of screw members are arranged to abut the troughs (or bottoms) of the annular leaf spring, it is possible by using screw members to give pressure to the outer ring of the bearing through the leaf spring, even if the outer ring of the bearing used in a motor is small due to the miniatuarization of the motor. Also it is a merit of such a structure that the leaf spring can serve as a buffer member. Furthermore, an engaging projection can be provided with either one of the inner periphery of the end bracket surrounding the pressure-giving structure receiving space and the annular leaf spring. Also an engaged portion such as a fitting groove or the like to be engaged with the engaging projection can be provided with the other of the inner periphery surrounding the pressure-giving structure receiving space and the annular leaf spring. The leaf spring can be positioned easily through engaging of the engaging projection and the engaged portion, resulting in an easy matching of the trough (or bottom) portions of the leaf spring and the screw holes. It is noted that other positioning structures may be used as well. In respect of a large-sized motor, tip end portions of the screws need not be arranged to abut the trough (or bottom) portions of the leaf spring. In this case, the tip end portions of the screw members may be arranged only to abut the outer ring of the bearing. A means for positioning the leaf spring is not necessary in this case.
One screw member can be used as a stopper. However if more than one screw members are used as a stopper, it is possible to prevent the moving of the second bearing more surely. In this case, it is preferable that the screw holes are arranged so as to surround a shaft through hole (or a through hole for the shaft) at almost equal interval in the circumference of the shaft. This construction allows a substantially equal pressing force to be applied to the second ball bearing from the trough portions of the leaf spring, resulting in that the moving of the second ball bearing is prevented, even when the adhesive layer has peeled off.
The stopper may also comprise through holes which extend through the second end bracket in the axial direction toward the first end bracket and more than one pin members which are forcedly fitted into the through holes and whose tip end portions abut either the trough (or bottom) portions of the leaf spring or the outer ring of the second ball bearing. When pin members are used, the pressing force may be set by initially deciding the length of the pin members which can provide a desired pressing force. To prevent pin members from coming off, either the pin members or the through holes may be tapered in such a manner that the radial dimension of either the pin members or the through holes become smaller toward the second bearing.
When the stopper comprises screw members or pin members and the screw holes or through holes and a leaf spring is used as a spring member, the motor will be manufactured as described in the following. After fixing the first end bracket to the stator body, uncured adhesive is applied between the inner periphery surrounding the first ball bearing fitting space in the first end bracket and outer surface of the outer ring of the first ball bearing, and the ball bearing coming-off preventing structure is set to work. Next, an annular leaf spring is arranged in a compressed state between the outer ring of the second ball bearing and the inner periphery of the second end bracket, and uncured adhesive is applied between the inner periphery of the second end bracket surrounding the second ball bearing fitting space and the outer surface of the outer ring of the second ball bearing. In this condition the second end bracket is fitted to the stator body. After uncured adhesive has cured, more than one screw members or pin members are inserted into screw holes or through holes to be fixed and the tip end portions of such screw members or pin members are arranged to abut trough (or bottom) portions of the leaf spring of the second ball bearing.